Modern aircraft typically include at least one, and usually two or more prime movers in the form of main engines which not only supply thrust for the aircraft but also power for numerous loads. In addition, an auxiliary power unit (APU) is typically provided on-board the aircraft and drives a generator. The APU is conventionally started by a DC motor and clutch arrangement which receives starting power from a 28 volt battery. Such a starting system has reliability and maintainability problems due to the DC motor brushes and the clutch.
In an attempt to eliminate these problems, the DC motor and the clutch arrangement have been omitted and power converter electronics have been added to operate the generator coupled to the APU as a motor to supply the required starting motive power to the APU. The power electronics include a step-up inverter to convert the 28 volt DC power into 150 volt DC power, an AC inverter to supply conditioned power to the main generator armature coils and an exciter inverter to provide field current to the main generator field winding via the generator exciter. While such a system works well, it has proved to be too costly.
A further start system has been devised utilizing two inverters, one of which directly converts 28 volt DC power to AC power for the main generator armature windings and another inverter for providing main generator field current via the exciter of the generator. Such a starting system is not completely satisfactory owing to the relatively low source voltage and due to the absence of means for increasing the voltage applied to the generator.